Method of and apparatus for controlling the operation of gas compression apparatus

ABSTRACT

A control system for regulating operation of gas compression apparatus operable on base mode. The discharge pressure of the gas from the compression plant is sensed and a signal related thereto is compared to a predetermined signal to produce a resultant control signal, operable to modulate a dump valve which governs the venting of excess discharge gas to the atmosphere. When the magnitude of the resultant control signal reaches a predetermined value, a second control signal is transmitted to cause the dump valve to assume a fully open position. The second control signal also causes a valve regulating flow of gas to the inlet of the compressor to assume a fully closed position. The compressor is thereby in an idle state.

United States Patent Pilarczyk et al. 1 51 June 5, 1973 54] METHOD OFAND APPARATUS FOR 3,411,702 11/1968 Metot et a1. ..415 27 x CO ROLLINGTHE OPERATION OF 3,424,370 l/l969 Law ..41s 27 x GAS COMPRESSIONAPPARATUS Inventors:

Assignee:

Filed:

Appl.

Karol Pilarczyk, Loudonville; Hans Pennink, Scotia, both of N.Y.

Carrier Corporation, Syracuse, N.Y.

Feb. 23, 1971 Int. Cl. ..F04b 49/02, F04d 27/00 Field of Search ..417/282, 295, 53, v

References Cited UNITED STATES PATENTS Primary Examiner-Carlton R.Croyle Assistant Examiner-Richard Sher Att0meyHarry G. Martin, Jr. andJ. Raymond Curtin [57] ABSTRACT A control system for regulatingoperation of gas compression apparatus operable on base mode. Thedischarge pressure of the gas from the compression plant is sensed and asignal related thereto is compared to a predetermined signal to producea resultant control signal, operable to modulate a dump valve whichgoverns the venting of excess discharge gas to the atmosphere. When themagnitude of the resultant control signal reaches a predetermined value,a second control signal is transmitted to cause the dump valve to assumea fully open position. The second control signal also causes a valveregulating flow of gas to the inlet of the compressor to assume a fullyclosed position. The compressor is thereby in an idle state.

6 Claims, 3 Drawing Figures 229 zasg 234 d 208 -231 T INVENTORS F I 2KAROL PILARCZYK- BY HANS PENNINK ATTORNEY i PATENTEDJUH 5 I975 SHEET 3BF 3 FIG. 3

INVENTORS KAROL PILARGZYK HANS PENNINK L ATTORNEY METHOD OF ANDAPPARATUS FOR CONTROLLING THE OPERATION OF GAS COMPRESSION APPARATUSBACKGROUND OF THE INVENTION In gas compression equipment, such as aircompression apparatus employing a number of air compressors or aircompression stages, wherein ambient air is compressed and delivered to areservoir or storage vessel, for use at a variable rate, there are twogeneral types of operation involved. The first type of operation iscalled base load operation. The compressor is controlled so as todeliver an output of compressed gas substantially consistent with ademand, variable within predetermined limits. As the compressed gasrequirements decrease, a dump or bypass valve disposed in communicationwith the discharge conduit of the compressor will operate to vent theexcess capacity to the atmosphere. The dump valve will be modulated tomaintain the desired discharge pressure. The second type of operation iscalled intermittent load operation. When operating on intermittent load,the compressor will either be delivering full capacity to the plant withthe dump valve completely closed or will not be delivering anycompressed gas to the plant, with the dump valve completely open. Withintermittent mode operation, there is no modulating the dump valve asdone when operating on base-mode.

During base load operation, if the load on the compression plantdecreases substantially for a period of time, the bypass or dump valvewill be open for a prolonged period of time to vent the excess dischargecapacity to the atmosphere. Such venting operation substantiallydecreases the overall efficiency of operation of the compressionequipment by maintaining the operating costs at such times equal to thecosts when the load on the equipment is materially higher.

It is therefore an object of the invention to provide a novel controlsystem to place compression equipment operating in base mode insubstantially an idle state when the load on the equipment falls below aminimum predetermined point.

SUMMARY OF THE INVENTION This invention relates broadly to the controlof the operation of gas compression apparatus and more particularly togas compression apparatus of the type employing a multistage axial orcentrifugal compressor driven by a prime mover. Still more particularly,this invention relates to a control system for use in gas compressionapparatus operable to place the apparatus in an idle state if the loadon the apparatus falls below a predetermined point, when the apparatusis operating on base mode.

The control system includes means for sensing the pressure of thedischarge from the compression apparatus and for sending a first signalrelated to the sensed pressure. First means are provided for receivingthe pressure signal and for also receiving a first predeterminedreference signal. The last mentioned means is further operable totransmit a resultant signal which is directly related to the pressureand predetermined reference signals.

The resultant control signal is transmitted to the bypass valve, tomodulate same to maintain a substantially constant discharge pressure.The resultant control signal is also transmitted to second means, and

when the magnitude of the control signal has reached a predeterminedvalue, said second means transmits a second control signal to cause thebypass valve to fully open and the inlet valve regulating the flow ofair to the inlet of the compression apparatus to close. The gascompression apparatus is placed in. an unloaded state.

When the second control signal is transmitted to the bypass valve andthe inlet valve, valve means is actuated to prevent the concurrenttransmission of the first resultant control signal.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 of the drawings illustrates inschematic a first portion of the control system of my invention;

FIG. 2 illustrates in schematic a second portion of the control systemforming a continuation of the first portion; and

FIG. 3 illustrates a fragmentary view of the upper left-hand portion ofFIG. 1, enlarged in the interests of clarity.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing thereis shown a gas compression system embodying the novel control.

The gas compression system 10 includes an air compressor 11 for thepurpose of providing compressed air to a storage vessel or reservoir foruse with pneumatic machinery, or similar applications where compressedair is utilized as an energy source. It will be appreciated there aremany installations requiring compressed air in such large quantitiesthat it is necessary to supply an air compression plant utilizing amultistage axial or centrifugal compressor which is represented bynumeral 11. Compressor 11 may have any number of desired stages toobtain the requisite quantity and pressure of air. For the purpose ofillustrating a representative machine, compressor 11 has three stagesrespectively, 12 which is the first stage and is in series with a secondstage 13, and a third stage 14. Compressor 11 is driven by a primemover, such as an electric motor, which is not shown. It should also beunderstood, intercoolers may be employed between the several stages asrequired to limit the temperature increase of the compressed air.

Inlet line 18 is provided to admit ambient air through an inlet throttlevalve 19 to first stage 12 of compressor 11. Compressed air isdischarged from the last stage of the compressor via discharge conduit20 into storage tank or reservoir 22. A suitable compressed airdischarge line (not shown) distributes air from reservoir 22 to desiredlocations at which it is employed.

In order to maintain the pressure of discharged air in reservoir 22 at apredetermined pressure, a dump or bypass valve 24 is employed todischarge compressed air from compressor 11 to the atmosphere when thepredetermined pressure in the reservoir 22 is satisfied. Dump valve 24may be operated either at a fully opened position or a fully closedposition, or it may be modulated, according to the characteristics ofthe mode of operation employed. For example, dump valve 24 is modulatedto maintain the desired pressure in reservoir 22 when the compressor isoperating on base mode. When the compressor is operating on intermittentmode, the dump valve is either fully opened or fully closed.

For purpose of illustrating a preferred embodiment, the control systemin accordance with our invention system, whether the compressor isoperating on basemode or on intermittent r'nod'e."l-Iowever it should bespecifically understood, the'fluid devices may be re-- placed by theirequivalent electrical, electronic or electro-p'neumatic components,without departing from the scope of our invention, except in such caseswhere the utilization of such fiuidic devices is claimed as being anessential feature of my invention.

The fluid devices of the control system are operated from a source ofpressurized air, the source either being from the discharge line ofcompressor 11 (as shown) or from an external source (not shown) or froma combination of these two.

The pressure signal in the preferred embodiment is obtained from a pointintermittent the compressor discharge and reservoir 22. The signal flowsthrough conduit 30 through a first port 31 of valve 32 (see FIG. 2).Valve 32 is a two position valve, that is the valve will either pass theentire signal supplied thereto or will completely block the continuedflow of the signal. The manner in which valve 32 is controlled shall bemore fully explained hereinafter.

The signal passing through valve 32 is supplied to a plurality of mainsupply branch conduits, respectively 33, 34, 35 and 36. It should beunderstood that pressure regulators well known to those familiar in theart may be installed at desired locations in the various branch conduitsto obtain desired control pressures therein.

INLET VALVE AND BYPASS VALVE REGULATION Communicating with supply branchconduit 33 are supply lines 37 and 38 respectively of fluid amplifiers39 and 40. Lines 37 and 38 provide a supply input signal of a pre-setmagnitude to each of the amplifiers. As shall become apparenthereinafter, amplifier 39 provides a control signal to regulate theoperation of inlet valve 19, and amplifier 40 provides a control signalto regulate the operation of bypass or dump valve 24.

Referring to FIG. 3, an enlarged view of the amplifi- 4 plied to summingjunction 44 via conduit 53. A second control signal is supplied to thesumming junction via conduit 54. The manner in which the control signalsare provided via conduits 53 and 54 shall be more fully explainedhereinafter. u

A third signal of a pre-setmagnitude is supplied thereto via line 47,fixed resistors 48 and 49, and line A fourth control signal is suppliedto summing junction 44 via line 52, having communication with inlet line18. Line 52 is located downstream of the inlet valve. The pressure ofthe air entering the compressor thus provides a proportional controlsignal to summing junction 44. As the pressure of the air in line 18increases the control signal will also increase. Since valve 19 isgenerally throttled, the pressure in lines 18 and 52 is generally lessthan atmospheric.

Amplifier 39 includes a first output 57 and a second output 61. Thecontrol signal passing through output 57 is exhausted to the atmospherethrough resistor 58. The control signal through output 61 passes throughbooster 62 and conduit 67 to regulate the operation of inlet valve 19.Booster 62 operates to increase the gain of the control signal by apredetermined amount. Booster 62 has a constant pre-set signal suppliedthereto from conduits 63 and 64. Conduit 64 communicates with mainsupply branch 33. The magnitude of the control signal to valve 19 variesin accordance with variations in the control input signals passingthrough summing junctions 44 and 42. The manner in which the inputsignals vary shall be more fully explained hereinafter.

Capacitors 65 and 66 are provided in lines 57 and 61 respectively toincrease the stability of operation of amplifier 39 by providing afeedback signal to each of the summing junctions. Capacitor 65 isconnected with summing junction 42' via line 59 and resistor 60;capacitor 66 is connected with summing junction 44 via line 68 andresistor 69.

Amplifier 40 has a predetermined supply signal provided thereto via line38. Summing junctions 91 and 99 are associated with amplifier 40.Summing junction 99 ers and their associated components is illustratedfor purposes of clarity. Amplifier 39 has a first control input 41 and asecond control input 42. The signal transmitted to input 41 is suppliedfrom main supply branch conduit 36, to a branch line 44. Line 44 has'afirst fixed resistor 43 and a variable resistor 41' disposed thereinupstream from input 41. Controlling the magnitude of the signal suppliedvia line 44 is temperature sensing mechanism 46 which is provided tosense the temperature of the air passing through inlet line 18.

As is well known to those skilled in the art, as the temperature of theair increases it is desirable to increase the volume of air entering thecompressor to maintain proper operating characteristics. Conversely, asthe temperature of the air decreases it is desirable to decrease thevolume of air passing to the inlet of the compressor. Temperature sensor46 operates to selectively increase or decrease the magnitude of thecontrol signal flowing to summing junction 42' and input 41, byselectively bleeding or closing off line which is in communication withline 44.

Associated with second input 42 is summing junction 44. Summing junction44 has a plurality of control signals supplied thereto. A firstcontro'lsignal is suphas a predetermined control signal supplied theretovia conduit 94 which is in communication with supply branch 36. Thesignal flows in conduit 94 through a first resistor 95, and then passesinto conduit 95 having a second resistor 98 disposed therein, intosumming junction 99 from whence it passes to a first input 100 ofamplifier 40. The control signal in conduit 94 also flows therefrom,through conduit 97, to a first three way pneumatically operated relay200. Variable resistor 96 is disposed between conduits 94 and 97 for areason to be explained more fully hereinafter.

Summing junction 91 has a first input thereto via conduit 92. Conduit 92is in communication conduit 92. Conduit 92 is iii-communication withconduit 56, which receives an output signal from a second pneumaticrelay 210.

Summing junction 91 has a second input thereto via line 120, which is incommunication with conduit 90. Conduit 90 is disposed between thecompressor dis charge and the reservoir to provide a signal equal todischarge pressure. As the load on the compressor system increases, thepressure signal decreases due to the increased flow from the reservoir.Conversely, as the load decreases, the pressure signal will increase,due to' the decreased flow from the reservoir and consequent increase inback pressure resulting therefrom. Resistors 114 and 115 in conduit 90make the signal passing to summing junction 91 proportional to dischargepressure.

Each of the summing junctions 91 and 99 have output signal feedbackcircuits similar to the feedback circuits of amplifier 39. The feedbackcircuit for summing junction 91 includes capcitor 105, conduit 106 andresistor 107. The feedback circuit for summing junction 99 includescapcitor 104, conduit 108 and resistor 109.

Amplifier 40 has a first control signal output 101 and a second controlsignal output 102 therefrom. Output 102 terminates in fixed resistor 103which exhausts to the atmosphere.

Communicating with output 101 is booster 66 which is connected toconduit 64 via line 65. Booster 66 is similar to booster 62 heretoforedescribed and operates to amplify the output signal delivered thereintovia conduit 101. Communicating with booster 66 is conduit 110. Conduit110 communicates with conduit 112 which provides the control signal fromthe amplifier via conduit 101, to bypass valve 24.

Disposed between conduits 110 and 112 is solenoid valve 113 which hasfirst and second operating positions. In a first position, the valveacts to communicate conduits 110 and 112; in its second position valve113 operates to bleed all the pressure from the diaphragm of bypassvalve 24 to place same in its normally open position. The manner inwhich valve 113 operates shall be explained more fully hereinafter.

Line 111 communicates with line 110 upstream of valve 113. Line 111supplies the control signal delivered from amplifier 40 to a first port221 of a third three-way pneumatic relay 220. Relay 220 includes spring223 which operates to bias valve member 224 towards port 225. Relay 220also includes port 226, which exhausts to the atmosphere. Valve 224operates to either close port 225 to prevent any exhausting of a controlsignal provided through lines 233 and 234, or alternatively, operates toopen port 225 to exhaust the control signal for a reason to be morefully explained hereinafter.

A predetermined control signal, provided through lines 36, 229, and 222,and resistor 228, provides an additional force to bias member 224 so asto prevent communication between ports 225 and 226. The variable controlsignal supplied via line 111 provides a force in opposition to the forcesupplied by spring 223 and the control signal from line 222, to movemember 224 towards port 227 to thereby exhaust the control signal fromline 234 through port 226.

Disposed upstream of the connection between line 229 and line 222 isresistor block 230. Resistor block 230 has a first line 231 and a secondline 232 which communicate with line 229. Variable resistor 237 isdisposed in line 232. The signal passing through line 232 and resistor237 is exhausted to the atmosphere.

Disposed in line 231 is a second variable resistor 238. Line 231 iscommunicated via variable resistor 238 with line 239. Line 239 connectswith switch 208 which has first and second operating positions dependingupon the mode of operation of the compressor, i.e., whether thecompressor is operating on either base or intermittent mode.

As shown, when switch 208 is in its dotted line position so that line55' is in communication with line 239, the compressor is operating onbase mode. Line 239 is thus exhausted to the atmosphere via line 55'.When the compressor is operating on intermittent mode, switch 208 isplaced in its solid line position so that line 55' is in communicationwith line 209.

The control signal in line 234 is provided via lines 233 and 290. Thesignal flowing through these two lines is also directed to line 235.Line 235 includes fixed resistor 236. Line 235 is connected to lines 229and 232. Relay 220 operates to selectively exhaust the control signal ofline 233 for a reason to be more fully explained.

Line 290 communicates line 233 with main supply branch 36. Line 290 hasa resistor 291 disposed therein and terminates at valve 113 to regulateoperation of the valve in a manner to be more fully explainedhereinafter.

Line 201 communicates line 290 with first three-way pneumatic relay 200.Line 201 provides a predetermined control signal through port 203 tobias valve member 207 so port 205 is closed and ports 204 and 206 are incommunication. Line 97 terminates at port 204. Spring member 202provides an opposed biasing force to move member 207 towards port 204 toprevent communication between ports 204 and 206. When ports 204 and 206are in communication, the control signal flows from line 97 to line 209having variable resistor 209 disposed therein.

Line 209 terminates at switch 208. When switch 208 is in itsintermittent mode of operation position, line 209 is communicated withline 55, to thereby exhaust the control signal in line 209 to theatmosphere.

Line 243 communicates line 290 with a fifth threeway pneumatic relay240. The control signal supplied via line 143 operates to bias member147 so as to close off port 242 of the relay. Opposing the forcesupplied by the control signal of line 243 is spring member 244. Whenthe spring force exceeds the predetermined control signal force, member247 is biased towards port 241 of the relay so as to communicate port242 with port 248, thereby exhausting a control signal in line 249 tothe atmosphere.

Line 249 has a predetermined control signal supplied thereto from line292, which has a fixed resistor 293 disposed therein. Line 292 is incommunication with main supply branch 36. The operation of relay 240shall be more fully explained hereinafter.

SURGE PROTECTION As is well known to those skilled in the art, it isdesirable to protect the compressor during surge conditions. The controlsystem in accordance with our invention achieves the desideratum byproviding a plurality of interconnected components automaticallyoperable to place the compressor in an unloaded state upon theoccurrence of surge.

Line 286 is connected to the inlet of the third stage of the compressorand thus supplies a pressure control signal to lines 283, 284 and 285.Line 284 has a variable resistor 281 disposed therein. The signalsupplied via line 284 is delivered to three-way pneumatic relay 270. Theforce supplied by the signal operates to move valve member 274 of therelay so as to communicate ports 277 and 278. Line 283 has a variableresistor 282 disposed therein and terminates by exhausting to theatmosphere.

Line 285 and line 405 supply the pressure signal to port 272 of relay270. The signal supplied through port 272 provides a force in oppositionto the force supplied by the signal through port 273. Spring 271provides an additional opposing force. Spring 271 and the control signalforce supplied through port 272 operate so as to keep valve member 274in its solid line position so as to prevent communication between ports277 and 278. Line 275 communicates port 277 with branch conduit 34 whichhas a predetermined control signal therethrough.

When member 274 is placed in its dotted line position in a manner to bemore fully explained hereinafter, the control signal from line 275passes through port 277 to port 278 and line 266.

Line 266 communicates port 278 with port 265 of an additional three-wayrelay 260. Port 265 is selectively communicated with port 269 by theoperation of valve member 264. When member 264 is in its solid lineposition, port 265 is in communication with port 269 so as to deliverthe control signal supplied via line 266 to line 288 which in turnsupplies the control signal to lines 287, 289 and 211.

Line 287 terminates in port 287 of relay 260. The control signalsupplied thereto provides a force so as to move member 264 to its dottedline position.

Acting to bias member 264 in its solid line position is spring 267, andthe force supplied via the control signal provided from line 261 throughport 262.

When member 264 assumes its dotted line position, port 263 of relay 260is in communication with port 269, thereby communicating lines 268 and288.

Line 268 is in communication with conduit 34 so as to provide apredetermined control signal to line 288, and lines 287, 289 and 211 incommunication therewith.

Line 289 delivers the control signal from line 288 to port 254 ofthree-way pneumatic relay 250. The signal provided thereby operates tobias valve member 252 of the relay towards its dotted line position.Opposing this is a force supplied by spring member 251. Spring mem ber251 operates to keep member 252 in its solid line position, whereby port255 is in communication with port 253. When member 252 is biased to itsdotted line position, port 255 is in communication with port 257'. Line256 communicates port 255 with conduit 34 so as to supply apredetermined control signal thereinto. When member 252 is in its solidline position, the control signal supplied via line 256 is passed toline 258 having resistors 259 and 259 disposed therein. This controlsignal is essentially exhausted through resistor 259 to the atmosphere.When member 252 is in its dotted line position, line 256 is incommunication, via ports 255 and 257', with line 257, so the controlsignal is transmitted to indicator light 403.

The control signal supplied via line 211 is delivered to port 213 ofthree-way pneumatic relay 210. This control signal provides a force tomove. valve member 214 to its dotted line position. Spring 212 providesa force to bias member 214 towards its solid line position. When member214 is in its solid line position, line 215 is communicated with theatmosphere via ports 216 and 217. When member 214 is in its dotted lineposition, port 216 is closed so as to prevent the control signal in line215 from being exhausted to the atmosphere.

Line 215 communicates line 257 with line 56, to provide a control signalto lines 53 and 92 respectively. The control signal is provided to theamplifiers to position the inlet and bypass valves in their normalpositions when surge conditions have been detected. The operation of thesurge detecting feature shall be more fully explained.

PROTECTION AGAINST UNDESIRABLE OPERATING CONDITIONS, EXCEPT SURGE Afurther feature of the control system includes means to prevent thecontinued operation of the compressor when a potentially harmfulcondition exists. For example, severe damage may occur if the compressoris operated when the temperature of the lubricating oil exceeds apredetermined value, or if the pressure of the oil diminishes below apredetermined value, or if the temperature of the air that is beingcompressed exceeds a predetermined value at some point in thecompression cycle.

The safety means of our control system comprise only fluidic devices.Heretofore similar safety means have included electroandelectro-pneumatic components. Such devices are subject to wear andtherefore require high maintenance costs. Sometimes, due to theirrelative inaccessibility, routine maintenance is eliminated, therebycreating a possibility that such devices may not function as requiredand resultant damage to the compressor may occur.

The fluidic devices in accordance with our invention are modular inconstruction and are easily removed from their mounting since a plug-infeature is included. Routing maintenance may then be readily conductedand if replacement of a module is required, such operation can bequickly conducted to reduce maintenance costs. In addition, the fluidicdevices generally do not wear as quickly as other devices, therebyfurther reducing maintenance costs.

Main supply branch 35 provides a predetermined control signal to lines411, 412, and 413, having communication therewith. The safety devicesheretofore noted operate so during normal operation, the control signalssupplied through lines 411, 412 and 413 are bled to atmosphere. When thecondition detected against occurs, the particular safety device thatsenses same operates to prevent the bleeding of air through theparticular line, for example the air temperature safety device operatesto prevent bleeding of air from line 411 upon the temperature of the airincreasing beyond a predetermined point. Line 411 communicates with line348 and port 343 of a three-way pneumatic relay 340. Relay 340 includesa second port 344 having communication with line 349, which transmits apredetermined control signal therethrough from line 338. Variableresistor 339 is disposed in line 338.

Fixed resistor 414 is disposed in line 411. Relay 340 includes valvemember 342 which when in its solid line position operates to communicateport 346 with port 347. The forces supplied by the predetermined controlsignal transmitted through port 344, and spring member 341, operate tomaintain member 342 in its solid line position.

The force supplied by the control signal transmitted via line 348,through port 343, operates to move member 342 to communicate port 345with port 347. Port 347 selectively exhausts the control signal fromlines 359 or 359' to the atmosphere. Line 359 is in communicat'ion withline 357, which in turn is in communication with main supply branch 35.Line 357 is additionally in communication with port 353 of three-waypneumatic relay 350. The force supplied by the control signal providedthrough port 353 operates to place valve member 356 of the relay intoits dotted line position. Spring 351 acts in opposition to the controlsignal force to bias the member so that it is in its solid lineposition, thereby closing off port 354. When member 356 is in its dottedline position a predetermined control signal is supplied via line 358,and ports 354, 355 of relay 350, to lines 417 and 418. The controlsignal supplied via line 417 is delivered to indicator light 402. Thecontrol signal passing through line 418 is delivered via line 420 toport 32 of valve 32.

Devices 300, 310, 320 and 330, are identical in operation and structureto relays 340 and 350. Thus they need not be explained in detail. Thedevices are actuated in response to the occurrence of some othercondition or conditions, such as those heretofore noted.

OPERATION The manner in which the control system functions to regulatethe operation of the compressor during normal operation and duringcertain adverse conditions will now be explained.

The regulation of inlet valve 19 in response to changes in the pressureand/or temperature of the ambient air entering the compressor will nowbe explained. The control of inlet valve 19 in accordance with thevariations in the condition of the entering air is identical regardlessof in which of the two modes of operation the compressor is operating.

As will be more apparent hereinafter, the control signals to summingjunctions 44' of amplifier 39, supplied by lines 53 and 54 do not affectoperation of the inlet valve in accordance with temperature and/orpressure variations in the inlet air. Accordingly, let us assume thatthere are no control signals being transmitted by either line 53 or line54. Therefore, the only control signal passing to summing junction 44 issupplied via conduits 47 and 52.

In order to understand the operation of the amplifier 39, let us assumethat one of the variables (pressure, temperature) of the air enteringthe compressor has remained constant and the other has changed. Forexample, let us assume that the pressure of the air entering thecompressor has remained constant and the temperature thereof has varied.

Since pressure of the entering air is assumed to have remained constant,the control signal input transmitted to summing junction 44 and to input42 of the amplifier also remains constant. If the temperature of the airincreases, temperature sensor 46 operates to increase the magnitude ofthe control signal flowing in line 44 to summing junction 42' and toinput 41. When the signal at input 41 exceeds the magnitude of thecontrol signal at input 42, the supply input provided through line 37 isbiased to agreater extent through output 61. There is a simultaneousdecrease in the passage of supply air through output 57. By increasingthe magnitude of the control signal flowing through output conduit 61and thence through booster 62, where the output signal is amplified byhaving the predetermined control signal supplied via lines 63 and 64combined therewith, a greater control signal is passed to the diaphragmof inlet valve 19. The increased magnitude control signal operates tofurther increase the inlet opening so that a greater volume of air ispassed to the first stage of the compressor.

Conversely, if the temperature of the air decreases, temperature sensor46 operates to decrease the magnitude of the control signal to input 41of amplifier 39, thereby decreasing the magnitude of the control signaleventually passing to the diaphragm of inlet valve 19 so as to diminishthe flow of air to the compressor.

Now let us assume that the temperature of the air remains constant andthat the pressure of the air varies, for example, the pressure of theair increases. As the pressure increases, a proportional increase willoccur in the control signal passing through conduit 52 which increasesthe control signal passing to summing junction 44' and input 42. Theincreased signal at input 42 biases the supply signal from conduit 37 sothat a greater amount thereof flows through output 57 and to theatmosphere via resistor 58. A decrease in the magnitude of the controlsignal passing through output 61 will thus occur, which will in turndecrease the magnitude of the control signal transmitted via conduit 67to the diaphragm of inlet valve 19, thereby modulating the valve todecrease the quantity of air passing to the compressor.

Conversely, if the pressure of the air were to decrease, an increasedmagnitude control signal will be felt at the diaphragm of inlet valve 19to modulate the valve towards an increased opening position to increasethe flow of air to the first stage of the compressor.

If both temperature and pressure of the air entering the compressorvaries, amplifier 39 will operate to compare the changes and to producea signal in output 61, which is proportional to the changes to properlyposition the inlet valve.

BASE LOAD OPERATION The manner in which the bypass valve and inlet valveare regulated, depending upon the mode of operation, will now beexplained. Assume that it is desirable to operate the compressor on baseload so bypass valve 24 is modulated to maintain a constant dischargepressure. Switch 208 is therefore placed in its base load position so asto communicate line 239 with line 55.

As noted hereinbefore, line provides a control signal which isproportional to discharge pressure. Line 90 is communicated with summingjunction 91 of control amplifier 40. As the discharge pressure increasesthe magnitude of the control signal increases. However, the signal isactually a negative input so that as the magnitude of the control signalincreases, a decreased biasing force is presented at input 93.Conversely, as the discharge pressure decreases so as to decrease themagnitude of the control signal, which is proportional thereto, anincrease biasing force will be presented at input 93.

The control signal supplied via line 94 and 95 provides a predeterminedcontrol signal to input of amplifier 40, via summing junction 99. Themagnitude of the control signal is regulated by the setpoint adjustmentobtained via variable resistor 96.

Assume the discharge pressure has increased to decrease the biasingforce at input 93. As the biasing force decreases, the magnitude of thecontrol signal in output 102 increases, with a concurrent decrease inthe magnitude of the signal in output 101. The decreased magnitudecontrol signal is transmitted from output 101 to booster 66 where thesignal is amplified. The signal then passes through line 110, valve 113and line 1 12 to the diaphragm of bypass valve 24. The decrease in themagnitude of the control signal biases the valve toward its normallyopen position, thereby increasing the flow of air to the atmosphere todecrease the discharge pressure as is desired.

Conversely, if the discharge pressure decreases below the predeterminedvalue, as determined by the signal to input 100, the change in forceacting on the supply input signal provided by line 92, will increase themagnitude of the signal at output 101 and decrease the signal at output102. The increased magnitude signal is transmitted to the diaphragm ofvalve 24 to modulate same to a more closed position to increase thedischarge pressure as desired.

If the load on the system served by the compressor is substantiallydecreased, the bypass valve will approach a fully open position toexhaust almost the entire output of the compressor to the atmosphere.Continued operation of the compressor in its normal fashion decreasesthe machines efficiency by increasing the operating cost, which may besubstantial if the load on the system remains small for a prolongedperiod of time. Therefore, one of the essential features of the controlsystem is to provide means for automatically placing the compressor inan unloaded state during base mode operation when a pre-selected rate ofbypass has occurred.

As noted hereinbefore, the control signal supplied to the diaphragm ofbypass valve 24 is also transmitted through line 111 to control relay220. As the control signal decays due to an increase in the dischargepressure, the predetermined control signal provided by line 229 throughport 228 and spring 223 operate to move member 224 so as to close offport 225. Until port 225 is closed by operation of member 224, thecontrol signal supplied through line 234 is exhausted via port 226. Line234 communicates with line 233 and thus with line 290. Resistor block230 having variable resistors 237 and 238 operates to continuouslyexhaust a portion of the control signal from line 233 through lines 235,231 and 232. Since the compressor is operating on base load, variableresistor 238 is exhausted to the atmosphere via line 239 and line 55. Bycontinuously exhausting a portion of the control signal to theatmosphere, the magnitude of the signal transmitted via line 229 to port228' is limited. It is therefore only upon a substantial decay in thecontrol signal provided by line 111, as a result of a large decrease inthe load on the compressor, that member 224 is actuated to close port225. With port 225 closed by member 224, less bleeding of the controlsignal transmitted by line 233 occurs, thereby increasing the magnitudeof the control signal in line 290.

A concurrent increase occurs in the control signal supplied via line 243to port 246 of relay 240. The increased control signal operates inopposition to the force supplied via spring 244 to move member 247 toits dotted line position so as to close off port 242. Heretofore, withmember 247 in its solid line position, the control signal supplied frommain supply branch 36 and line 292 to line 249 has been bled via ports242 and 248. With port 242 closed, the control signal in line 249 istransmitted to line 54 having communication with summing junction 44 ofamplifier 39. The increased biasing force thus placed on the supplysignal, eliminates the signal through output 61, to place inlet valve19in its unloaded or normal position, that is, fully closed.

Additionally, the control signal in line 290 is passed to valve 113 soasto place same in its second operating position, whereby the controlsignal from line is prevented from passing to line 112. In addition, the

control signal already operating on the diaphragm of bypass valve 24 isexhausted to the atmosphere, thereby placing the valve in its normallyopen position so the compressor is fully in an unloaded state.

Thus, it is apparent that with the novel control system, the controlautomatically operates so as to place the compressor in an unloadedstate when the machine is operating on base mode and the dischargepressure has exceeded a predetermined magnitude due to a substantialdecrease in the load on the machine. This results in a substantialsaving in operating costs. When the load on the machine increasesthereby decreasing the discharge pressure, the control automaticallyoperates to place the compressor in normal base mode operation.

It should be noted with switch 208 in its base load position, line 209is effectively blocked, thereby making relay 200 inoperative during baseload operation.

INTERMITTENT LOAD OPERATION Assume now instead of operating thecompressor on base mode the machine is operating on intermittent mode.For such operation, the compressor is operated in an unloaded stateexcept when the discharge pressure falls below a minimum predeterminedpoint. Switch 208 is placed in its intermittent mode position,communicating line 209 with line 55, and preventing any exhausting ofair through resistor 238 and line 239. Thus there is an increasedbiasing force placed on member 224 to maintain same in its dotted lineposition, due to the increased control signal supplied to port 228 vialine 222.

As is apparent, it will only require a small decay in the control signalsupplied via line 111 to port 221 for member 224 to assume its dottedline position so as to close port 225. With port 225 closed, the controlsignal supplied via line 290 is increased very rapidly in the samemanner as heretofore explained for base mode operation.

The signal is transmitted from line 290, via line 201, to actuate valvemember 207, to switch it to its dotted line position therebycommunicating ports 204 and 206. The signal transmitted via line 97 isthus exhausted to the atmosphere through line 209, valve 208 and line55. By immediately exhausting the predetermined control signal providedto summing junction 99 of amplifier 40, the amplifier senses a falseexcessively high discharge pressure due to the complete absence of anypredetermined control signal. By sensing the false excessive dischargepressure, the supply signal is biased completely to output 102, tothereby remove any pressure from the diaphragm of bypass valve 24, toplace same in its normally open position. The manner in which inletvalve 19 is placed in its normally closed position is the same asheretofore explained for base mode operation.

SURGE PROTECTION OPERATION The manner in which the control systemoperates to protect the compressor during surge conditions will now'beexplained.

Line 286 communicates line 283, 284 and 285 to the inlet of the thirdstage of the compressor. If surge occurs, the pressure in line 285decreases rapidly during each cycle of reverse flow of air through thecompressor. However, such rapid decrease of pressure will not occursimultaneously in conduit 284 due to having resistor 281 disposedtherein, thus a pressure differential is established between ports 272and 273 of relay 270. Due to the high pressure at port 273, member 274will move to its dotted line position, opening up port 277 tocommunicate line 275 with line 266 via port 278. Line 266 communicateswith port 265 of relay 260, thus the predetermined control signaltransmitted from conduit 34 flows through ports 265 and 269 to lines287, 288, 289 and 211. By providing the predetermined control signal toline 287, valve member 264 of relay 260 moves to its dotted lineposition, thereby opening port 263 to communicate line 268 with line288. By communicating lines 268 and 288, the control system is lockedinto its surge detecting state until otherwise reset by means to be morefully explained hereinafter.

The predetermined control signal from line 288 passes into line 289having communication with port 254 of relay 250. This predeterminedcontrol signal supplies a force of a sufficient magnitude to cause valve252 of the relay to move to its dotted line position, to communicateline 257 with port 255 of line 256. Until valve member 252 is soactuated, line 256 and port 255 have been in communication with port 253and line 258 having resistors 259 and 259 disposed therein. Resistor259' exhausts the predetermined control signal of line 256 to theatmosphere.

When valve 252 closes port 253, the predetermined control signal istransmitted by line 257 to indicator light 403, to actuate same toindicate a surge condition has occurred.

The predetermined control signal from line 268 additionally passes toline 211 having communication with port 213 of relay 210. Thepredetermined control supplies a force to cause valve member 214 to moveto close port 216, to prevent the signal in line 215 from beingexhausted to the atmosphere. Line 215 is in communication with line 257so the control signal transmitted by line 257 is also transmitted byline 215 to line 56 in communication therewith. Line 56 is incommunication with line 53 which provides the predetermined controlsignal to summing junction 44' of amplifier 39. By providing thepredetermined control signal to the summing junction, the supply signalthereto is biased towards output 57 to eliminate the control signal fromthe diaphragm of inlet valve 19 so as to place same in its normallyclosed position.

Line 56 is also in communication with line 92 which communicates withsumming junction 91 of amplifier 40. By providing the predeterminedcontrol signal to summingjunction 91, which as noted hereinbeforeprovides a negative input to the amplifier, the supply control signalprovided through line 100 is biased towards output 102. The controlpressure acting on the diaphragm of valve 24 is eliminated to place thevalve in its normally open position. When surge occurs, the compressoris placed in an unloaded state to prevent any damage to the compressor.

In order to take the compressor out of its unloaded state, an operatorhas to manually push reset valve 370 to communicate line 30 with line371. The predetermined control signal thus passes through resistor 372to lines 373 and 261, communicating with port 262 of relay 260. Thepredetermined control signal supplies a force, to assist the springforce, to move valve 264 to close port 263.

With port 263 closed, the predetermined control signal is prevented frompassing from line 268 to lines 287, 288, 289, and 211. Thus, if a surgecondition no longer exists, there will be no pressure signal providedthrough line 266 so the operation of the compressor may return tonormal, and the inlet and bypass valves will be operated as heretoforeexplained, depending on the mode of operation.

OPERATION UPON THE OCCURRENCE OF AN UNDESIRABLE OPERATING CONDITION,OTHER THAN SURGE The manner in which the components of the controlsystem operate to effectively discontinue operation of the compressorwhen a potentially harmful condition occurs will be now explained.

As noted hereinbefore, the sensing devices installed to detect theoccurrence of some unwanted condition operate, when in their normalstate, so as to bleed lines 411, 412, and 413. Let us assume the airtemperature of the compressed gas at some particular point, such asdownstream of the intercooler (not shown) between the second and thirdstages has exceeded a safe value. The particular sensing device operatesto close the bleed port of line 411. Line 411 is in communication withmain supply branch 35. When the bleed port of line 411 is closed, apredetermined control signal is supplied from line 35, via line 411, toline 348 and port 343 of relay 340. The force provided by the controlsignal operates to move valve member 342 so as to open port 345 in line359 and to close port 346 in line 359. Before the actuation of the valvefrom its solid line position, port 346 has been in communication withport 347 so as to exhaust the signal in line 357, transmitted theretofrom line 35. By opening port 345, to communicate same with exhaust port347, the control signal previously supplied port 344, from lines 349 and359, is exhausted to the atmosphere, thereby diminishing the forceacting in opposition to the force supplied by the control signaldelivered through port 343.

By closing port 346, the predetermined control signal in line 357 issupplied to port 353 of 350. The force supplied by the predeterminedcontrol signal operates to actuate valve member 356 to move same towardsport 354. Simultaneously port 354 in line 358 is uncovered to provide apredetermined control signal therefrom to port 355.

The predetermined control signal then flows through lines 417 and 418.Line 418 has a check valve disposed therein to permit flow only fromport 355 to line 420 in communication with line 418. The control signalthus provided passes to port 32' of air operated valve 32 so as toactuate the valve to prevent any flow from line 30 to line 33 incommunication therewith. By eliminating any control air, the inlet andbypass valves are placed in their normally closed and open positionsrespectively.

In addition, the control signal may be supplied to a fluidic pressureswitch (not shown) in the electrical circuit of the prime mover, to stopthe prime mover. Valve 32 is so formed that once it has been placed ineither of its two operating positions, it does not change position untila positive force is exerted on its opposed port. For example, even ifthe temperature of the air diminishes to normal, the compressor may notbe restarted until an operator manually resets the valve so as to permitcommunication between lines and 33. It is thus apparent that thecompressor fails safe, that is any operation of the compressor iscompletely prevented once an unwarranted condition has occurred, therebyeliminating any possible human error.

Line 417 provides the control signal to a particular indicator light toindicate the source of trouble. The control signal is then exhausted vialine 371 which includes resistors 372 and 382. Resistor 382 exhaustssubstantially all of the control signal to the atmosphere. Any controlsignal supplied via lines 417 and 371 to port 32" is not of sufficientmagnitude to move the valve to communicate lines 30 and 33.

When the operator wishes to restart the compressor, reset valve 370 ismoved from its normally closed position, to a position so as tocommunicate line 30 with line 371, thereby placing a sufficient controlsignal on port 32" of valve 32 so as to move same to its open position,whereby lines 30 and 33 are communicated. Normal operation of thecompressor may then be resumed.

The novel control system herein disclosed effectively prevents operationof the compressor during conditions which may render any furtheroperation unsafe to the compressor. In addition, the novel controlsystem effectively operates the compressor so as to provide a moreefficient operation to reduce the operating costs of the compressor.

While we have described and illustrated a preferred embodiment of ourinvention, it will be understood that the invention is not limitedthereto but may be otherwise embodied within the scope ofthe followingclaims.

We claim:

1. A control system for automatically placing gas compression apparatusin an unloaded state during base mode operation, when the load on saidcompression apparatus has decreased below a predetermined level, saidcompression apparatus, including a gas compressor, an inlet conduit,valve means governing flow of gas through the inlet conduit to thecompressor, means for storage of gas delivered by the compressor,discharge means connecting the compressor and the storage means, andbypass valve means for venting the discharge of the compressor to theatmosphere comprising:

a. means for sensing the pressure of the gas flowing through saiddischarge means and for creating a first signal related to saiddischarge pressure;

b. means for receiving a first signal of a predetermined magnitude andsaid signal relating to said discharge pressure and transmitting aresultant control signal;

0. first means for receiving said resultant signal and operable tomodulate said bypass valve meansto maintain a substantially constantdischarge pressure;

d. second means for receiving said resultant control signal and operablewhen the magnitude of said signal has reached a predetermined value dueto the load on said compression apparatus having decreased below apredetermined level to send a second control signal to cause said bypassvalve means to assume a fully open position and to further cause saidinlet valve means to assume a fully closed position; and

e. means operable to prevent said first resultant control signal frombeing transmitted to said bypass valve means when said second controlsignal is being transmitted, said bypass valve means thereby beingmaintained in a fully open position so long as the load on saidcompression apparatus remains below said predetermined level.

2. A control system in accordance with claim 1 wherein said lastmentioned means includes valve means having first and second operatingpositions, said valve means transmitting said first resultant controlsignal when in its first operating position and being further operableto exhaust any signal from said bypass valve means when in its secondoperating position, said valve means being placed in its secondoperating position upon transmission of said second control signal.

3. A method of regulating the operation of gas compression apparatusoperating on base mode, whereby said apparatus is automatically placedin an unloaded state when the load on the apparatus has decreased belowa predetermined level, said apparatus including a gas compressor, aninlet conduit, a valve governing the flow of gas through the inletconduit to the compressor, means for storage of gas delivered by thecompressor, discharge means connecting the compressor and storage meansand bypass valve means for venting the discharge of the compressor tothe atmosphere comprising: 7

a. sensing the pressure of the gas flowing through the discharge means;

b. generating a first signal relating to said sensed discharge pressure;

0. receiving the signal relating to said discharge pressure andcomparing same to a signal of a predetermined magnitude to produce aresultant control signal;

d. transmitting the resultant control signal through a first flow pathto modulate the bypass valve means to maintain a substantially constantdischarge pressure; and

e. transmitting the resultant control signal through a second flow path,when the magnitude of said resultant signal has reached a predeterminedvalue due to the load on the compression apparatus having decreasedbelow a predetermined level whereby a second control signal is generatedto cause said bypass valve means to assume a fully open position andsaid inlet valve means to assume a fully closed position, said bypassand inlet valve means being maintained in said positions until the loadon said compression apparatus has increased above said predeterminedlevel.

4. The method in accordance with claim 3 further comprising the step ofblocking the transmission of the resultant control signal through saidfirst flow path when the magnitude of the signal has reached thepredetermined value.

5. A control system for automatically placing a gas compressionapparatus in an unloaded state during base mode operation when the loadon such compression apparatus has decreased below a predetermined level,said compression apparatus including; a gas compressor, an inletconduit, valve means covering flow of gas through the inlet conduit tothe compressor, means for storing gas delivered by the compressor,discharge means connecting the compressor and the storage means, andbypass valve means for venting the discharge of the compressor to theatmosphere comprising:

a. means for sensing the magnitude of the load on said gas compressionapparatus;

b. means for generating a first control signal, the magnitude thereofvarying in response to changes in said load;

0. means for receiving said first control signal and being operable togenerate a resultant control signal, the magnitude thereof varying inresponse to changes in load on said compression apparatus; and

(1. means operable to receive said resultant control signal, when theload on said compression apparatus has decreased below a predeterminedlevel, the magnitude of said resultant control signal being at a valuewhereby said resultant signal receiving means is operable to place saidcompression apparatus in an unloaded state wherein the inlet valve meansis closed and said bypass valve means is open, thereafter when the loadon said compression apparatus has increased above said predeterminedlevel, the magnitude of said resultant control signal will vary wherebysaid resultant signal receiving means is further operable to place saidbypass and inlet valves in their loaded base mode states.

6. The method of placing gas compression apparatus operating on basemode in an unloaded state when the load on such compression apparatushas decreased below a predetermined level, said compression appara tusincluding; a gas compressor, an inlet conduit, valve means governingflow of gas through the inlet conduit to the compressor, means forstoring the gas delivered by the compressor, discharge means connectingthe compressor and the storage means, and bypass valve means for ventingthe discharge of the compressor to the atmosphere comprising the stepsof:

a. sensing that the load on said compression apparatus has fallen belowa predetermined level;

b. regulating the bypass valve means so it is placed in a fully openposition and the inlet valve means so it is placed in a fully closedposition when the decrease in the load has been sensed, to place thecompression apparatus in an unloaded mode of operation;

c. maintaining the inlet and bypass valves in their unloaded mode solong as the load on said compression apparatus remains below thepredetermined level; and

d. sensing that the load on said compression apparatus has increasedabove said predetermined level to thereby place said valves in theirloaded base mode states

1. A control system for automatically placing gas compression apparatusin an unloaded state during base mode operation, when the load on saidcompression apparatus has decreased below a predetermined level, saidcompression apparatus, including a gas compressor, an inlet conduit,valve means governing flow of gas through the inlet conduit to thecompressor, means for storage of gas delivered by the compressor,discharge means connecting the compressor and the storage means, andbypass valve means for venting the discharge of the compressor to theatmosphere comprising: a. means for sensing the pressure of the gasflowing through said discharge means and for creating a first signalrelated to said discharge pressure; b. means for receiving a firstsignal of a predetermined magnitude and said signal relating to saiddischarge pressure and transmitting a resultant control signal; c. firstmeans for receiving said resultant signal and operable to modulate saidbypass valve means to maintain a substantially constant dischargepressure; d. second means for receiving said resultant control signaland operable when the magnitude of said signal has reached apredetermined value due to the load on said compression apparatus havingdecreased below a predetermined level to send a second control signal tocause said bypass valve means to assume a fully open position and tofurther cause said inlet valve means to assume a fully closed position;and e. means operable to prevent said first resultant control signalfroM being transmitted to said bypass valve means when said secondcontrol signal is being transmitted, said bypass valve means therebybeing maintained in a fully open position so long as the load on saidcompression apparatus remains below said predetermined level.
 2. Acontrol system in accordance with claim 1 wherein said last mentionedmeans includes valve means having first and second operating positions,said valve means transmitting said first resultant control signal whenin its first operating position and being further operable to exhaustany signal from said bypass valve means when in its second operatingposition, said valve means being placed in its second operating positionupon transmission of said second control signal.
 3. A method ofregulating the operation of gas compression apparatus operating on basemode, whereby said apparatus is automatically placed in an unloadedstate when the load on the apparatus has decreased below a predeterminedlevel, said apparatus including a gas compressor, an inlet conduit, avalve governing the flow of gas through the inlet conduit to thecompressor, means for storage of gas delivered by the compressor,discharge means connecting the compressor and storage means and bypassvalve means for venting the discharge of the compressor to theatmosphere comprising: a. sensing the pressure of the gas flowingthrough the discharge means; b. generating a first signal relating tosaid sensed discharge pressure; c. receiving the signal relating to saiddischarge pressure and comparing same to a signal of a predeterminedmagnitude to produce a resultant control signal; d. transmitting theresultant control signal through a first flow path to modulate thebypass valve means to maintain a substantially constant dischargepressure; and e. transmitting the resultant control signal through asecond flow path, when the magnitude of said resultant signal hasreached a predetermined value due to the load on the compressionapparatus having decreased below a predetermined level whereby a secondcontrol signal is generated to cause said bypass valve means to assume afully open position and said inlet valve means to assume a fully closedposition, said bypass and inlet valve means being maintained in saidpositions until the load on said compression apparatus has increasedabove said predetermined level.
 4. The method in accordance with claim 3further comprising the step of blocking the transmission of theresultant control signal through said first flow path when the magnitudeof the signal has reached the predetermined value.
 5. A control systemfor automatically placing a gas compression apparatus in an unloadedstate during base mode operation when the load on such compressionapparatus has decreased below a predetermined level, said compressionapparatus including; a gas compressor, an inlet conduit, valve meanscovering flow of gas through the inlet conduit to the compressor, meansfor storing gas delivered by the compressor, discharge means connectingthe compressor and the storage means, and bypass valve means for ventingthe discharge of the compressor to the atmosphere comprising: a. meansfor sensing the magnitude of the load on said gas compression apparatus;b. means for generating a first control signal, the magnitude thereofvarying in response to changes in said load; c. means for receiving saidfirst control signal and being operable to generate a resultant controlsignal, the magnitude thereof varying in response to changes in load onsaid compression apparatus; and d. means operable to receive saidresultant control signal, when the load on said compression apparatushas decreased below a predetermined level, the magnitude of saidresultant control signal being at a value whereby said resultant signalreceiving means is operable to place said compression apparatus in anunloaded state wherein the inlet valve means is closed and said bypassvalve means is open, therEafter when the load on said compressionapparatus has increased above said predetermined level, the magnitude ofsaid resultant control signal will vary whereby said resultant signalreceiving means is further operable to place said bypass and inletvalves in their loaded base mode states.
 6. The method of placing gascompression apparatus operating on base mode in an unloaded state whenthe load on such compression apparatus has decreased below apredetermined level, said compression apparatus including; a gascompressor, an inlet conduit, valve means governing flow of gas throughthe inlet conduit to the compressor, means for storing the gas deliveredby the compressor, discharge means connecting the compressor and thestorage means, and bypass valve means for venting the discharge of thecompressor to the atmosphere comprising the steps of: a. sensing thatthe load on said compression apparatus has fallen below a predeterminedlevel; b. regulating the bypass valve means so it is placed in a fullyopen position and the inlet valve means so it is placed in a fullyclosed position when the decrease in the load has been sensed, to placethe compression apparatus in an unloaded mode of operation; c.maintaining the inlet and bypass valves in their unloaded mode so longas the load on said compression apparatus remains below thepredetermined level; and d. sensing that the load on said compressionapparatus has increased above said predetermined level to thereby placesaid valves in their loaded base mode states.